The present invention relates to the field of brushless DC motor control, and more particularly to a method and system for determining a magnetic offset of a brushless DC motor and for using the magnetic offset to provide a commutation current that allows the motor to operate at its highest efficiency.
Typically, brushless DC motors comprise a fixed stator with a number of windings and a moveable rotor with a set of magnets. Current passing though the windings, referred to as the commutation current, results in the production of an electromagnetic field. The field causes movement of the rotor magnets and an output force to be supplied by the motor. For the motor to perform efficiently, the commutation current used to provide excitation of the windings must be of the proper phase and value. A motor that is not supplied with the proper commutation current will be inefficient in converting current to force or torque, and may even operate in reverse.
To discover the amount and phase of the appropriate commutation current, the location of the rotor magnets with respect to the windings must be determined. A position sensor first determines the initial location of the magnets in relation to the poles of the motor. For incremental position sensors, the measured position value is an arbitrary distance when the sensor is initialized. For proper commutation, it is imperative that an offset value (i.e., the incremental distance) be determined so that the sensor position can be aligned to the actual poles of the magnets. This offset value is referred to as the "magnetic offset."
One method for aligning the position sensor to the actual location of the poles is to supply a three-phase current value to the windings using an arbitrary magnetic offset. The supplied current causes the motor to rotate with a certain force and in certain direction, depending upon the initial alignment of the windings and the magnets and the phase separation of the current supplied. The current supplied is a pulse and the same pulse is supplied in the opposite direction to avoid significant motor movement. Subsequent three-phase current pulses are then supplied, either lower or higher than the initial current setting, depending on the movement and force output by the motor. Excitation of the windings with the arbitrary successive magnetic offset values continues until a maximum force is output by the motor. Once the maximum force is achieved, the offset is saved for future commutation.
This procedure requires short current pulses to avoid unwanted motor movement. These pulses can not be too short, however, because measurement errors effecting the accuracy of the magnetic offset may result.
Offset determination is important because the force produced by the motor is a sinusoidal function of how accurately the magnetic offset is calculated. For example, if the error between the calculated and real magnetic offset is 3%, then the force produced by the motor will reach no higher than 98% of its maximum possible (and expected) force output. For an error of 7% the force output by the motor falls to 90%. FIG. 1 is a waveform representation of the relationship between a magnetic offset error and the force output by a sinusoidally commutated motor. As apparent in FIG. 1, the closer the magnetic offset error is to zero, the higher the force produced by the motor and the more efficiently it operates.
As such, it is desirable to reduce the magnetic offset error between the position detected by a sensor monitoring the distance between the magnets and windings, and the actual offset between the magnets and the windings. It is also desirable to quickly and accurately determine the magnetic offset so that a proper commutation current can be supplied to motor. It is further desirable that the determination of the magnetic offset occur with minimal movement and stress upon the motor. This determination should take place automatically without the need for continued monitoring of the power output by the motor.
It is therefore desirable to provide a solution that effective overcomes the problems and disadvantages associated with the related art.